Acta Zoologica Cracoviensia, 26 (1982 - 1983)
Shell Microstructure and Systematics of Sphaeriidae (Bivalvia, Eulamellibranchiata)
Abstract:
Seventeen species of the Sphaeriidae have been included in this study. In the members of this family the periostracum forms about 5% of the shell thickeness and on its inner and outer surfaces it bears sculpture, which is presumably a characteristic feature of particular species. Spherical or elongated conchiolin structures were observed on the inner surface of the periostracum of Pisidium nitidum; they strengthen its adhesion to the calcareous layers of the shell. The calcareo-organic portions of the shell are differentiated into layers distinguished by their homogeneous-granular, granular (homogeneous, Kobayashi, 1971), diagonal (composite prismatic, Kobayashi, 1971) and palisade (pallial myostracum) structures. In most species of the family studied canals, perpendicular to the shell surface and directed towards the inside of the shell, were seen in the calcareous layers. An analysis of microstructural characters indicates that the presence or lack of homogeneo-granular and granular structures, the value of angles between the lamellae of the diagonal layer, the shape of plates of the palisade layer and the pores are of essential taxonomic importance. The microstructural characters support the division of the Sphaeriidae, generally applied at present, and add new data to the issue of the systematic position of several taxa, e.g. P. casertanum f. ponderosum and P. nitidum f. crassa.
